A variety of approaches are known for dissipating heat generated by power semiconductor devices. One such method entails the use of a ceramic substrate, such as alumina (Al.sub.2 O.sub.3) or another ceramic material that may be modified to promote its heat conduction capability. Heat-generating integrated circuit (IC) chips, such as power flip chips, are often mounted to alumina substrates that conduct and dissipate heat in the vertical direction away from the chip. These designs are limited in their ability to dissipate heat laterally because the thermal conductivity of ceramic materials is low compared to metals and metal-containing materials, though relatively high compared to printed circuit boards (PCBs). Therefore, thick-film conductors are often used with power IC components on alumina substrates to promote lateral heat conduction away from the component. The typical thermal performance for power IC flip chips mounted in this manner is about 6 to 12.sub.-- C rise per Watt of power dissipated. Disadvantages with ceramic substrates include the relatively higher cost of the substrate, and ceramic materials do not lend themselves to the use of leaded or through-hole mounted components such as relays, large diodes and transistors. In addition, components generally cannot be mounted to both sides of a ceramic substrate.
Another known method for thermal management of a power flip chip is to use a flex circuit laminated or bonded to heatsink formed by an aluminum sheet or layer having a typical thickness of about 0.05 inch (about 1.25 mm). If the flex circuit material is sufficiently thin, e.g., less than about 0.25 mm, this approach can employ a highly conductive path formed by copper-plated via holes through the flex circuit to the aluminum heatsink. The typical thermal performance for power IC flip chips mounted on flex circuits in this manner is about 3 to 12.sub.-- C/Watt power dissipated. However, the entire rigid section of the circuit must be backed with aluminum, which makes it difficult to mount leaded parts and prevents components from being mounted to the side of the substrate with the heatsink.
Yet another method for dissipating heat from a power IC is to package the device and mount it to a heat rail on a PCB. Typical thermal performance for power IC's mounted in this manner is about 2 to 4.sub.-- C/Watt power dissipated. However, the packages and heat rails are relatively large in size, and heat rails must generally be specially fabricated to mount IC packages. Power packages with more than fifteen pins and that can be mounted to a heat rail are generally not available or are very expensive.
Finally, PCBs have been equipped with innerlayer heatsinks that are the same size or are larger than the PCB to provide a large heatsink for the entire board, and rely on conduction through the PCB material to the heatsink beneath. However, this thermal management approach is relatively costly, has not been used with PCBs carrying flip chips, and is not compatible with small electrical vias, resulting in reduced wiring density.
From the above, it can be seen that the various current approaches to thermal managing power IC's have drawbacks that limit their application, incur significant additional costs, and/or impose undesirable restrictions on circuit layout and design. Therefore, it would be desirable if a circuit construction were available that provided improved thermal management for power ICs, and particularly power flip chips. Such a structure would preferably permit the use of power flip chips of minimal size, enable thermal enhancement of localized areas of the circuit substrate for lower cost and greater design flexibility, be compatible with leaded components, and permit mounting of components on both sides of the circuit substrate.